For reasons of environmental protection, solvent-borne coating systems are increasingly being displaced by aqueous systems. In many cases, however, the advantageous properties of the known solvent-borne systems are not yet matched by the aqueous coating materials.
In EP-B 0 000 605, aqueous curing agents are disclosed for aqueous epoxy resin dispersions which are prepared by reacting adducts of polyalkylene polyether polyols and water-insoluble polyepoxide compounds with polyamines whose reactive groups have been subjected to addition reaction with unsaturated compounds capable of such reaction.
In EP-B 0 387 418, a simplification of the difficult synthesis of a polyether-modified resin precursor is described which starts from amino-terminated polyether glycols. In a second stage—completely in analogy to EP-B 0 000 605—the precursor is reacted with an excess of polyamines to form adducts.
EP-A 0 567 831 relates to aqueous curing agents composed of an emulsifier which is prepared from an amino-terminated polyalkylene polyether, an aliphatic diglycidyl ether, and a fatty amine; a coemulsifier prepared from a polyamine, an aliphatic diglycidyl ether, and a fatty amine; and also a bis-amine adduct formed from epoxy resin and a polyamine.
In combination with commercially available aqueous dispersions of epoxy resins, these known systems give coatings which are in need of improvement with respect in particular to their corrosion resistance.
It is also known that reaction products of epoxy resins with phosphoric acid or with organic phosphonic acids can be processed to aqueous coating compositions which lead to coatings having good corrosion resistance. Aqueous resins of this kind, however, are cured with curatives which only take effect at a relatively high temperature (baking systems).
Amine-type curatives containing phosphorus can be formulated by reacting epoxide-amine adducts with phosphoric acid. The product of this reaction is a crystal slurry which can be managed on the laboratory scale and which after a certain time, with vigorous stirring, becomes a viscous mass. A reaction of this kind is highly problematic on the industrial scale, however, since the stirrer may come to a standstill and there may be local overheating.
The object is therefore to provide phosphorus-containing amine-type curatives for aqueous epoxy resin dispersions that can be prepared without problems on the industrial scale and which in aqueous systems can be formulated together with aqueous epoxy resins to give coating compositions which cure at room temperature or only slightly elevated temperature and which lead to enhanced corrosion protection.
This object is achieved by reacting partly esterified phosphoric acids or organic phosphonic acids with amine-type curatives selected from epoxy-amine adducts, Mannich bases, polyamidoamines, and condensation products of diamines or polyamines with di- or polyhydroxyaromatics, with addition reaction and/or salt formation. Throughout the reaction the reaction mass is always in the form of a homogeneous, liquid, viscous system which can be managed without problems.